Micro-optical electromechanical system (MEOMS) membranes are used in a spectrum of optical applications. For example, they can be coated to be reflective and then paired with a stationary mirror to form a tunable Fabry-Perot (FP) cavity/filter. They can also be used as stand-alone reflective components to define the end of a laser cavity, for example.
The MEOMS membranes are typically produced by depositing a membrane structure over a sacrificial layer, which has been deposited on a support structure. This sacrificial layer is subsequently etched away to produce a suspended membrane structure in a release process.
Often the membrane layer is a silicon compound and the sacrificial layer can be polyimide, for example.
Typically, membrane deflection is achieved by applying a voltage between the membrane and a fixed electrode on the support structure. Electrostatic attraction moves the membrane in the direction of the fixed electrode as a function of the applied voltage. This results in changes in the reflector separation of the FP filter or cavity length in the case of a laser.
There are advantages associated with using crystalline membrane layers when making mechanical structures. They tend to be mechanically stable, not being susceptible to creep, for example, or other long-term degradation or changes in their optical, mechanical, or electromechanical performance. These characteristics are generally desirable in MOEMS-type devices, especially devices that are to be deployed in carrier-class systems since long term stability is an important metric for characterizing such devices.
In general, according to one aspect, the invention features a process for fabricating an optical membrane device. This process comprises providing a handle wafer and then oxidizing a surface of the handle wafer to form an insulating layer. A device wafer is then bonded to the handle wafer. An optical membrane structure is formed in this device wafer. The insulating layer is selectively removed to release the membrane structure. This device wafer can be manufactured from silicon wafer material. Such material typically has a low number of dislocations yielding a stable mechanical membrane structure.
Another important issue in MOEMS design concerns layer thickness. The design of such layers typically have optical as well as electromechancal constraints. For example, optical membranes must typically deflect a distance corresponding to the wavelength of light on which they operate. They are usually electrostatically deflected, however, and operating voltages can be limited by air ionization factors and voltages that are available from the systems in which they operate.
In general, according to another aspect, the invention concerns an optical membrane device. This device comprises handle wafer material. This functions as the mechanical support for the device. An optical membrane layer is provided in which a deflectable membrane structure is formed. An insulating layer separates the handle wafer material from the optical membrane layer. The insulating layer defines the electrical cavity, across which electrical fields are established that are used to electrostatically deflect the membrane structure. According to the invention, the insulating layer is between 3 and 6 micrometers (xcexcm) in thickness.
In the preferred embodiment, the insulating layer is greater than 3 micrometers in order to ensure that xe2x80x9csnap downxe2x80x9d, i.e., the unintentional contact between the membrane and another part of the device, is avoided. So that the device, however, can be operated with voltages with less than approximately 50 to 100 volts, the insulating layer is preferably less than 5 micrometers in thickness. From this information, the corresponding rigidity of the membrane is selected.
Concerning the preferred embodiment, the handle wafer material is preferably between 100 and 1000 micrometers in thickness. An optical port can also be provided, through the handle wafer, to enable direct optical access to the membrane structure without having to traverse through the wafer material. Preferably, to yield a proper balance between structural stability and deflectability, the optical membrane layer is between 5 and 10 micrometers in thickness. Presently, it is between 6 and 8 micrometers in thickness.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.